The invention relates to a method and equipment for making the static filtration media disclosed in U.S. application Ser. No. 09/506,575 filed Feb. 18, 2000, the disclosure of which is hereby incorporated by reference herein. For example the invention is useful in the manufacture of the following preferred embodiment of the substrate from the application Ser. No. 09/506,575:
Polyester, or other fiber, non-woven fabric about 4 to 7 ounces/sq. ft. when compressed and about 8 mm thick
Functional Coating equal to about 100 to 200% of the uncoated fabric weight, comprised of:
FDA compliant vinyl acrylic binder about 10 to 20% of the coating by weight
Coconut shell activated carbon about 60 to 85% of the coating by weight
Zeolite molecular sieve about 0 to 25% of the coating by weight, preferably about 5-20%
A useful term to describe a filter medium, which can be operated in a static manner, is the ratio of xe2x80x9creadily deliverable fluid volumexe2x80x9d (RDV) to total bed volume (BV). Readily deliverable fluid volume is defined here as the volume of fluid, which will drain from a decanted filter bed without the application of any external force (other than gravity). Static filters typically exhibit RDV/BV ratios from 30 to 80 percent, as measured from the cessation of streaming flow.
Traditional filtration devices cannot be operated effectively in a static manner, because the extra-particle bulk volume in a packed bed is very small relative to the bed volume. The RDV/BV ratio of a granular activated carbon bed packed with 12xc3x9730-mesh carbon is typically 9 percent for a cylindrical bed around 8.5 inches in depth and 4.5 inches in diameter. The argument cannot be made that a packed bed overlaid with a column of fluid constitutes static treatment, as the mean distance between a fluid molecule and an adsorptive site is too large to allow for treatment within a reasonable amount of time. In addition, in such a system the tortuosity of the fluid path between the particles of the packed bed would hinder diffusion to the point of making the majority of the bed inaccessible to adsorption.
Compression is used to reduce the average voids which hold the water, to be approximately 6-7 (e.g. about 6.5) *10xe2x88x928 liters in volume. This equates to a RDV/BV ratio in the neighborhood of 63% for a cylindrical bed around 8.5 inches in depth and 4.5 inches in diameter, as measured from the cessation of streaming flow. Larger voids are tolerated if residence time between use is not a priority. Controlling the RDV/BV ratio rather than the minimum fill and pour rate, addresses a weakness inherent in EP 0402661.
A significant improvement in the treatment material described in EP 0402661 is obtained by a post impregnation modification of the substrate. The capacity of the media is significantly increased by narrowing the void size distribution about what is believed to be an optimum value. Fill and pour rates are determined to a large degree by the size of the largest voids in the non-woven. Capacity on the other hand is determined by smaller relative percentages of the smallest voids, since capacity is determined by the ability of the fluid to freely flow from the media and not be held by capillary forces. (Capacity in the context of this application refers to the volume the filter is able to deliver on demand, rather than amount of contaminant removal which can occur over the life of the filter). In effect what is desired is to have a medium which has a narrow size distribution of voids just large enough to maintain the rapid fill and pour rates. By employing a base material which has a void size distribution in a range where the smallest voids are excluded, compression can be used to preferentially collapse the larger voids. This yields a product with the narrowest void size distribution, centered around an optimal size value for production capacity, flow rate, and removal rate. This effectively alleviates a static-filter size restriction which exists if the fabric contains a large number of small voids, and creates an improvement in efficiency over a product which contains larger volume voids. Without this technique it becomes more difficult to fabricate a static filter which can still pour effective volumes when in a small size configuration.
The diffusion of material from inside a well mixed sphere of fluid to it""s sorptive outer boundary can be modeled by             ∂      C              ∂      t        =      D    ⁡          (                                                  ∂              2                        ⁢            C                                ∂                          r              2                                      +                              2            r                    ⁢                      xe2x80x83                    ⁢                                    ∂              C                                      ∂              r                                          )      
where C is the concentration within the fluid, t the time the fluid has been within the sphere, D the diffusion coefficient of the chemical dissolved in the fluid, and r the radial dimension within the sphere. A mathematical transformation facilitates the solution of this problem.
By setting
u=Cr
the differential mass balance (Equation 1) becomes             ∂      u              ∂      t        =      D    ⁢          xe2x80x83        ⁢                            ∂          2                ⁢        u                    ∂                  r          2                    
which is easily solved[1] for the ratio between the total mass of chemical which has left the sphere at a given time and the amount that would eventually be removed             m      t              m      ∞        =      1    -                  6                  π          2                    ⁢              xe2x80x83            ⁢                        ∑                      n            -            1                    ∞                ⁢                  xe2x80x83                ⁢                              1            n                    ⁢                      xe2x80x83                    ⁢                      exp            ⁡                          (                                                                    -                                          Dn                      2                                                        ⁢                                      π                    2                                    ⁢                  t                                                  a                  2                                            )                                          
where mt is the mass removed at time t, m∞ the mass which would be removed at equilibrium, and a the radial dimension at the sphere surface. 
FIG. 1: The theoretical effects of compression on performance.
It will be seen from the results of this model (Equation 1) that the size and distribution of void spaces within the filter media has a marked affect on the performance of the filter with respect to contaminant removal. If one assumes a Weibull distribution of void sizes with parameter xe2x80x98axe2x80x99 of 20 and a parameter xe2x80x98bxe2x80x99 of 1.05, the kinetics of removal are shown in Table 1 and FIG. 1. In this figure we see that by shifting just the largest 10% of voids to the median value of the previous distribution by selectively collapsing some of the larger void spaces, we obtain a significant improvement in the amount of contaminant removed in a given time. Table 1 lists the percentage removal for the theoretical system at several time intervals. If the treatment objective is to reach 90% removal of a particular contaminant (as is required for certification by the NSF[2] for lead removal), the figure illustrates that this improvement may be dramatic in terms of ease of use of the filter. In this example the uncompressed media takes twice as long to reach compliance.
Laboratory analysis of media performance under various levels of compression were performed to validate the theoretical model, with these results presented in Table 2. A controlled volume vessel was packed with media compressed from 100% to around 50% of it""s original thickness. Media which was midway through it""s useful life was used for testing at exposure times of 3 minutes, in order to yield effluent concentrations which were detectable by anodic stripping voltametry. The performance of the media for lead removal was shown to increase dramatically with compression, moving from a low tested value of 84% removal at 12% compression to a maximum tested value of 96% at 50% compression.
The preferred manner of manufacture of coated non-wovens such as those which can be used in static filtration, are to draw rolls of a web (e.g. non-woven fabric) through a dipping bath where the materials the fabric is to be coated with are suspended. For purposes of the invention the bath contains water treatment materials exemplified by: activated carbon, ceramic cation-exchangers such as zeolites, amorphous gels such as sodium aluminosilicate or sodium titanium silicate, or polymers utilizing contaminant specific ligands; as well as a binder to secure them to the web, e.g. non-woven fabric. The fabric is generally pulled through the bath where the fabric becomes saturated with coating material, and then through a series of rollers which squeeze excess coating from the fabric for return to the dipping bath. As the coated fabric exits the rollers it is then pulled or pushed through a drying oven or other curing zone where the binder is allowed to cure. Tension is usually maintained at the leading and trailing ends of the fabric to ensure that the fabric moves through the process in a uniform manner, and other agents may be added to the coated fabric prior to drying in order to facilitate curing the binder.
The preferred substrate for the fabric is polyester, due to its wettability and stability. Since the filters are designed for use in treating potable water, a substrate which is listed under Title 21 of the Code of Federal Regulations, Section 177.2260 (21 CFR 177.2260) is appropriate. The adsorbent material used to coat the substrate is typically ground to a powder in order to facilitate the coating process and improved the kinetics of adsorption, but static treatment media can be produced using particles of essentially any size so long as the RDV/BV requirements are not violated. The polyester base is preferentially formed into a non-woven fabric prior to coating using the same FDA compliant binder which is used to coat the fabric. Also (a)-(c) may be practiced with a web speed between about 2 and 35 ft/min (or any other narrower range within that broad range).
According to one aspect of the invention there is provided a method of treating a web capable of water treatment having a first void size distribution, comprising: a) coating the web with a coating composition that facilitates treatment of water brought into contact therewith and includes a binder; b) substantially simultaneously with or after a), compressing the web in one dimension about 25-75% (preferably 40-60%, e.g. 50%) so as to make the void size distribution substantially more uniform; and c) substantially maintaining the compression of the web from b) until the binder substantially cures so that the coated final web produced has a second void size distribution more uniform than the first void size distribution, and water treatment facility.
In the method c) may be practiced so as to produce a coated final web having void volumes with a mean value of about 6-7xc3x9710 to the minus 8th liters, and to produce a coated final web having an RDV/BV ratio of at least 0.4, and a porosity of greater than 90%. In one embodiment a) is practiced using a non-woven web having a weight of about 4-7 ounces/sq. ft., and applying a coating equal to about 100-200% of the uncoated weight of the fabric comprising about 10-20% binder, about 60-90% (e.g. 60-85%) activated carbon, and about 0-25% (e.g. 5-20%) zeolite.
In the method b) may be practiced by substantially immediately after a) passing the web through a series of staggered rollers. Alternatively, a compression of about 40-60% may be practiced by tightly winding the web prior to cure of the binder; or b) may be at least primarily (e.g. substantially exclusively) practiced by compressing the web in a curing tunnel, for example using upper and lower compression belt roller assemblies. In the method (a)-(c) may be practiced with a web speed of about 2-35 ft/min (and all narrower ranges within that broad range).
According to another aspect of the invention there is provided apparatus for producing a water treatment web, comprising: A feed roll of a web of material having a first void size distribution. A coating tank having a plurality of rollers therein through which the web from the feed roll passes, and applying a water treatment coating composition and binder to the web. A compression device which compresses the web (e.g. after exiting the coating tank) about 40-60% in a first dimension. A curing zone to allow at least partial curing of the web; and a take-up roll for taking up the final coated, compressed web.
The compression device may comprise a plurality of staggered rollers positioned substantially immediately after the coating tank; and the compression introduced by the compression device is preferably maintained substantially throughout the curing zone. Alternatively, the compression device and the curing zone comprise upper and lower compression belt roller assemblies in a curing tunnel, with squeeze rolls located above the coating tank and between the coating tank and curing tunnel for squeezing excess coating material from the web back into the coating tank. As another alternative the compression device primarily comprises the take-up roll, and wherein the take-up roll is positioned so as to take-up and compress the web after partial curing but prior to complete curing of the binder.